add cbegin() and cend() for explicit const-iterating, too., as common in C++11

pc_container.hh

@@ -36,6 +36,8 @@ public:
     
     typedef std::list<PC> Container;
     
+    typename Container::const_iterator cbegin() const noexcept { return c.cbegin(); }
+    typename Container::const_iterator cend()   const noexcept { return c.cend();   }
     typename Container::const_iterator begin()  const noexcept { return c.cbegin(); }
     typename Container::const_iterator end()    const noexcept { return c.cend();   }
     typename Container::iterator       begin()        noexcept { return c.begin(); }